Assets: you store these on your own directory. Since you have a NAS you want to use, they’d probably be stored there with the server. You don’t have to create from scratch, but you do need to possess them in some form. Rules: the game system should set up roll calculations and everything up right. There are no character creation sheets for 5e built, but I don’t know about other game systems. That being said, there are many modules and they can always be added to different effects. Server: On the home page for Foundry there are “worlds”. Each world is its own game. You can only have one up at a time but you can have as many as you have disk space for. For wanting to just plop down a map, yeah that can be done quick and easy.